This new R03 proposal explores the mechanisms through which the cGMP-dependent protein kinase I alpha (PKGI) inhibits aging-related cardiac remodeling in vivo. It is based on extensive new and unpublished data performed in the PI's laboratory. Preliminary studies have explored the cardiac remodeling response in a mouse model harboring discrete mutations in the PKGI leucine zipper interacting domain (the PKGI leucine zipper mutant, or LZM, mouse). In the baseline state, the LZM mice develop progressive, age-related left ventricular hypertrophy (LVH), a key component of remodeling. In the setting of LV pressure overload, a common risk factor in elderly patients, the LZM mice develop increased LVH, worsening contractile function, and striking accelerated mortality, compared with wild type littermate controls. These data reveal a novel role of PKGI in inhibiting aging-related cardiac remodeling in vivo. We have further identified early blunting of the anti-remodeling JNK pathway in hearts of LZM mice after TAC, compared with WT TAC hearts, and have identified a novel interaction of PKGI with the upstream MAPKKK and JNK activator mixed lineage kinase 3 (MLK3). These preliminary data support a mechanism by which PKGI, via interaction with MLK3, activates JNK to inhibit aging-related cardiac remodeling. Based on these, and other, preliminary data, this application proposes to test the central hypothesis that PKGI and its downstream targets in the cardiac myocyte serve as novel targets to treat aging-related cardiac remodeling. We propose to test this hypothesis through three specific aims, using a number of mouse models. In SA1 we will explore the mechanisms by which PKGI inhibits age-dependent LVH and remodeling in vivo, by examining the cardiac structure and function of a new cardiac myocyte-specific PKGI knockout mouse (PKGIcKO) created in our laboratory. We will also isolate cardiac myocytes (CMs) from PKGIcKO mice to explore the cellular mechanisms by which PKGI inhibits remodeling. In SA2 we will explore the response of the PKGIcKO mice to LV pressure overload induced by TAC, and will test the degree to which CM specific PKGI deletion abolishes the anti-remodeling effect of the phosphodiesterase 5 inhibitor sildenafil. In SA3 we will explore the cardiac remodeling response in a whole body MLK3 knockout mouse (MLK3-/-) to test the clinically relevant hypothesis that the PKGI-interacting protein MLK3 inhibits aging-related remodeling in vivo. These proposed studies will define the specific mechanisms through which PKGI in the CM inhibits cardiac remodeling. This project is highly significant because it directly addresses a problem which is highly prevalent in the elderly and because it directly tests new therapeutic targets. It is highly innovative because it uses unique in vivo models, and also because it employs an innovative strategy of exploring CM-specific regulators of cardiac remodeling, which could translate into novel, CM-specific treatments.